1. Field of the Invention
The present invention relates to strain sensors and particularly to an in-line fiber etalon strain sensor and method for making such in-line fiber etalon strain sensor.
2. Description of the Related Art
A variety of fiber-optic sensors for "smart" materials and structures applications have been proposed for monitoring the thermomechanical state of load bearing structural materials. Fabry-Perot (FP) type sensors have been extensively studied, including the extrinsic Fabry-Perot interferometer (EFPI) sensor (shown in FIG. 1), in which an air gap between two cleaved fiber end faces serves as the cavity, and the intrinsic Fabry-Perot interferometer (IFPI) sensor (shown in FIG. 2), which utilizes semi reflective or partially reflective surfaces on the end faces of a fiber "spacer" serving as the FP cavity.
The EFPI sensor of FIG. 1 is fabricated by inserting two cleaved single mode fibers 11 and 13 into a small diameter hollow tube or sleeve 15 for mechanical stability, and then bonding them in place by applying epoxy adhesive 17 to the edges 16 of the tube 15. The cavity 18 is defined by the space between the end-faces 19 and 21 of the respective fibers 11 and 13. Reflections R1 and R2 from the end-faces 19 and 21 produce an interference pattern which is proportional to the strain applied to the cavity 18.
The IFPI sensor of FIG. 2 is fabricated by coating both end faces of a section of single mode fiber 23 with a reflective layer or film to form reflective interfaces 25 and 27. Then the fiber 23 is fusion spliced at the reflective interface 25 end to a long single mode fiber 29. The section of single mode fiber 23 with the reflective interfaces 25 and 27 forms the cavity for the IFPI sensor of FIG. 2. Another long single mode fiber 31 may be fusion spliced to the other end of the single mode fiber 23 which contains the reflective interface 27. The reflective interfaces 25 and 27 thus become part of in-fiber reflective splices. One or more additional IFPI sensors may be added to the sensor system of FIG. 2 in the same manner as described above.
Both of the IFPI and EFPI sensor configurations have shown promise for structurally-embedded applications due to their ease of fabrication, lead-insensitivity, and high strain resolution. In addition, the EFPI is insensitive to transverse strains and has low thermal apparent strains. However, both the EFPI and IFPI sensors have at least two disadvantages that may limit their utility. First, their mechanical strength is significantly less than that of a typical optical fiber. Sensor strength is an important issue for embedded applications where the fabrication process of the structural member can lead to catastrophic failure of the fiber. Since embedded sensors are not easily repaired or replaced, refabrication of the component part is usually required.
In the case of the EFPI, there are stress concentrations due to diameter discontinuities between the lead-in fibers and the oversleeved alignment tube. Although the IFPI sensor is relatively immune to stress concentrations due to its continuous diameter, the degraded strength of partially reflecting splices may lead to early mechanical failure. A second concern for the EFPI is that the gage length is determined not by the air gap between the fibers but by the separation of the attachment edges 16 to the oversleeve tube. The epoxy used as a sealer wicks into the space between the tube and the fibers, so that the gage length for each sensor must be individually calibrated. Furthermore, microcracking of the epoxy from repeated cyclical loading will cause the gage length to change with time.
Nevertheless, the EFPI holds several advantages over the IFPI. The thermal optic coefficient of the IFPI sensor leads to significant ambiguity (thermal apparent strain) when strain measurements are made in thermo-mechanical loading environments, while the transverse strain insensitivity and low thermal apparent strain of the EFPI are particularly important for fiber optic "smart structure" applications where the sensor is commonly embedded within load-bearing structural components.